The program concerns the comparative studies of the processes of calcification of various invertebrates which form different types of calcium carbonate crystals - calcite, aragonite, and vaterite. The organisms will include Crassostrea virginica and other molluscs, coelenterates (Renilla reniformis and others), and echinoderms. The crystal type is different in different anatomical portions of the shells or skeletons in some organisms. In others, the type is changed during the development from the larval stage to the adult, or during regeneration. Also hydroxyapatite formation in teleost fish scales will be investigated. Transmission and scanning electron microscope observations will be carried out to correlate the cellular structures of the shell-forming and skeletogenic tissues to the formation of a particular type of crystals. Special emphais will be given to the processes of organic matrix synthesis, nucleation and growth of crystals, and pathways of calcium from the cellular organelles to the sites of calcification. At the same time, amino acid composition of the organic matrix will also be analyzed. Bibliographic references: Meenakshi, V.R., Blackwelder, P.L., Hare, P.E., Wilbur, K.M., and Watabe, N. 1975. Studies on shell regeneration-1. Matrix and mineral composition of the normal and regenerated shell of Pomacea paludosa. Comp. biochem. Physiol. 50A:347-351. Watabe, N. 1974. Crystal growth of calcium carbonate in biological systems. J. Crystal Growth 24/25: 116-122.